1. Field of the Invention
The present invention relates to a time discrete PLL-tuning system, comprising a phase detector and a voltage-controlled oscillator (VCO) for tuning the frequency (fVCO) thereof to a frequency equal to N/M times a reference frequency (fREF), with M being a factor indicating the number of frequency steps in which a transmitter/receiver channel distance is divided, and N being the number of frequency steps in which the oscillator frequency is divided.
2. Description of the Related Art
Such a tuning system is generally known and is applied in radios, tv""s, mobile telephones, etc. In the phase detector, the difference between the phase of an input signal with a predetermined frequency step (fSTEP) and the phase of a signal derived from the VCO output signal by dividing the frequency thereof with the factor N, is detected, filtered and fed back to the VCO. When the frequency step (fSTEP) is equal to a reference frequency (fREF) divided by the factor M, the VCO frequency is tuned to N/M times said reference frequency, i.e., to the frequency (N/M)*fREF. The reference frequency may be generated by a crystal oscillator.
The accuracy of the frequency of the tuned oscillator, on the one hand, and the switching speed to switch the oscillator between different frequencies, on the other hand, are limited as a consequence of the low sampling rate of the control loop which locks the tuned oscillator frequency to a frequency proportional to the fixed reference frequency. The best choice of this sampling rate, or sampling frequency, is the frequency step fSTEP of the tuned oscillator, this frequency step being usually a fraction (1/M) of the channel distance in telecommunication systems, i.e., fSTEP=(1/M)*fREF. In that case, in practice, the bandwidth of such a the tuning system will be about fSTEP/10 (instead of the theoretical Nyquist criterion fSTEP/2). For high accuracy, the frequency step and, therefore, the sampling frequency would be low. This requirement results in a small bandwidth of the sampled control system. However, the small bandwidth, in turn, results in slow switching speeds between different channels. The switching time from one channel to another will be about 1/fSTEP; the exact value depends on the step size and accuracy. Further, due to the sampling mechanism of the control loop, during tuning, a spurious component with a frequency equal to the frequency step (fSTEP=fREF/M) will always be present in the control loop. This means that a frequency modulated VCO signal will be obtained with the consequence that signals will be received from a transmitter transmitting in a neighboring channel at a frequency distance fSTEP from the frequency of the VCO. To keep the spurious signal sufficiently low, heavy loop filtering has to be applied. However, such a loop filtering results in a small bandwidth of the control system and, thus, to a slow switching speed.
Further in an article of Amr N. Hafez and M.I. Elmasry, presented at the IEEE 1999 Custom Integrated Circuits Conference, a wideband tuning system based on a dual (nested) FLL/PLL architecture is described with a PLL as described above. There, the FLL (frequency-locked loop), comprising a frequency-to-voltage feedback loop, integrated in the PLL, has a large bandwidth, a high reference frequency and low accuracy, while the PLL has a small bandwidth, a low reference frequency and high accuracy. In this way, the function of small step size/high accuracy and the function of large loop bandwidth are separated. Due to this separation, however, a disadvantage with respect to fast switching from one channel to another is introduced because the exact frequency is controlled only by the slow PLL. While the VCO frequency is tuned to the frequency step (indicated as reference frequency in said article), the FLL provides for a suppression of the phase noise. The FLL presented in this paper is based on a frequency-to-voltage converter which is critical with respect to high frequency operation. In order to apply this circuit, the clock frequency (spurious) of the FLL has to be reduced via a frequency divider, compared to the high frequency of the VCO. That means that the fundamental advantage of the FLL cannot be fully exploited.
It is an object of the invention to provide a time discrete PPL-tuning system wherein the disadvantages of the above tuning systems are avoided, this tuning system being accurate and having a high switching speed to switch the oscillator between two different frequencies.
Therefore, according to the invention, the time discrete PLL-tuning system is characterized in that the sampling frequency of the phase detector is substantial equal to the reference frequency (fREF). In the generally known PLL-tuning systems, the sampling frequency is, as mentioned above, equal to fSTEP; according to the invention, the sample frequency is M*fSTEP. This means that the spurious component has a frequency M*fSTEP which can be chosen far beyond the range of present neighboring transmitter/receiver channels, so that a wideband tuning system can be obtained and, thus, a rapid switching speed, while the frequency step can still be low, resulting in a high tuning accuracy. The bandwidth of the PLL-tuning system, according to the invention, is about fREF/10, which is a factor fREF/fSTEP=M wider than the bandwidth of the generally known PLL-tuning system.
Such a high sample frequency may be realized in a PLL-tuning system in which the phase detector comprises a first frequency-to-voltage converter, which, in response to the VCO signal, supplies an output signal (kxe2x80x2*fVCO) proportional to the oscillator frequency (fVCO), a multiplier unit which, in response to said output signal (kxe2x80x2*fVCO) of the frequency-to-voltage converter, supplies a signal (k*M*fVCO) proportional to the product (fVCO) of the oscillator frequency and a factor M, indicating the number of frequency steps in which a transmitter/receiver channel distance is divided, a reference signal unit which, in response to a reference frequency signal, supplies a signal (k*N*fREF) proportional to the product of the reference frequency (fREF) and a factor N, N being the number of frequency steps in which the oscillator frequency is divided, and a difference circuit supplying a signal proportional to the difference of the product of the oscillator frequency (fVCO) and the factor M and of the product of the reference frequency (fREF) and the factor N, this difference signal (k*(M*fVCOxe2x88x92N*fREF)) being subjected to low-pass filtering and supplied to the VCO as a control signal. This means that the VCO control signal Vc can be represented by:       V    C    =      K    *    M    *          ∫                        (                                    f                                                xe2x80x83                                VCO                                      -                                          N                M                            ⁢                              f                REF                                              )                *                  ⅆ          t                    
This relation will be the same as can be derived for the generally known PLL""s mentioned before. The important difference is that, in this embodiment, in spite of a low frequency step, the sample frequency is much higher than in the generally known systems, so that, in this embodiment, VSPURIOUS is formed by a signal with frequency fREF, while in the known PLL""s, VSPURIOUS is formed by a signal with frequency fREF/M.
The signals M and N may be supplied in digital form and are preferably adjustable. In such an embodiment, the multiplier unit may comprise a first DAC (digital-to-analog converter) with a current output to convert the signal M into a current, and a first current switch controlled by the output signal of the first frequency-to-voltage converter. Further, in order to obtain an substantially identical electronic structure, in such an embodiment, the reference signal unit may then comprise a second frequency-to-voltage converter, and a second DAC (digital-to-analog converter) with a current output to convert the signal N into a current to be supplied to a second current switch, this second current switch being controlled by the output signal of the second frequency-to-voltage converter.
In a preferred embodiment, each of the first and second frequency-to-voltage converters comprises an exclusive-OR circuit to which the signal with the oscillator frequency and with the reference frequency, respectively, are supplied directly and after a delay (xcfx84), and low-pass filter means for filtering the output signal of the exclusive-OR circuit. The delay (xcfx84) may be adjustable, which is important to obtain an accurate substantially equal adjustment of both values of the delay (xcfx84) in both frequency-to-voltage converters.